Continuous centrifugal tube casting with dry mold and gas pressure differential

ABSTRACT

THE INVENTION RELATES TO A RAPID AND IMPROVED MEANS FOR THE CONTINUOUS CENTRIFUGAL CASTING OF METALLIC AND NON-METALLIC TUBE ON TEH INTERIOR SURFACE OF A ROTATING SOLID-WALL BY UTILIZING NOVEL TECHNIQUES TO GREATLY REDUCE THE SIDE-WALL FRICTION.

Sept. 20, 1971 Q R, LEGHORN 3,605,859

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United States Patent O 3,605,859 CONTINUOUS CENTRIFUGAL TUBE CASTINGWITH DRY MOLD AND GAS PRESSURE DIFFERENTIAL George R. Leghorn, 1423Washington Ave., Apt. 1, Santa Monica, Calif. 90403 Continuation-impartof application Ser. No. 538,506, Feb. 11, 1966, now Patent No.3,445,922, dated May 27, 1969. This application Oct. 21, 1968, Ser. No.769,017

Int. Cl. B22d 11/00, 13/02, 27/16 U.S. Cl. 164-62 13 Claims ABSTRACT OFTHE DISCLOSURE The invention relates to a rapid and improved means forthe continuous centrifugal casting of metallic and non-metallic tube onthe interior surface of a rotating solid-wall by utilizing noveltechniques to greatly reduce the side-wall friction.

This application is a continuation-in-part of my application Ser. No.538,506, filed Feb. 11, 1966, now Pat. No. 3,445,922, issued May 27,1969.

BACKGROUND OF INVENTION A great many techniques for the casting of tubesare known and used in the metal casting industry and most of thesetechniques have long been in public domain. More than this, it has longbeen obvious that a means of continuously casting such tubing wouldpermit great economy to be realized in the manufacture of suchhollow-ware.

One of the earliest attempts for the casting of tubing on a continuousbasis is exemplified in British Patent 15,912 issued to Lane andChamberlain in 1891. This invention attempted to continuously casttubing by the expedient of continuously pouring the molten metal (to becast) into one end of a solid-wall centrifugal mold and continuouslyremoving the solidified tube from the other end. The technique, whilemeritorious in conception, failed primarily as a result of the highfrictional contact between the mold bore I.D. and the cast tube O D.

A number of other patents teach the continuous centrifugal casting oftube in a solid wall mold by the basic technique of Lane andChamberlain. These include U.S. Pats. 777,559 to 777,562 issued toStravs and Jager in 1904 (this series of patents disclosed bothhorizontal and vertically downward extraction of the tubes so cast);U.S. Pat. 950,884 issued to Winner in 1910 (in this method, asuperimposed slinging action was utilized to continuously force thecentrifugally cast tube from the mold bore); U.S. Patent 1,223,676issued to De Lavaud in 1917 which teaches the use of a rotary mold and aroller disposed within said mold as well as a means for continuouslyejecting the casting as formed; U.S. Patent 2,752,648 issued to Robertin 1956 (this is essentially a repeat of the methods of Strauss andlager as taught in their patent disclosures in 1904 and utilizes cantedrolls to extract the centrifugally cast tube downwardly from a verticalmold); and, lastly, British Pat. 984,053 issued in 1963 which teachesthe downwards extraction of a centrifugally cast tube from a verticalcentrifuge having an internal offset and tapered rotating core mold.

Whereas the foregoing processes have been made to work and producetubing in a continuous manner, they have the drawback of exceptionallyhigh frictional forces between the mold wall I.D. and the cast tube O D.as a result of the outward forces on the molten and solidified tubemetal due to the centrifugal action. Conventionally, horizontalcentrifugal casting is done between rotational speeds which produce from50 to 100 gravities of cen- 3,605,859 Patented Sept. 20, 1971 trifugalforce (a one pound mass of metal would effectively weight 50 pounds whencentrifuged at the rotational speed of 50 Gs) necessary to produce adense sound casting and to prevent raining and sloshing of the moltenmetal and, as a result, the extraction of the continuously centrifugallycast tube from the bore of the solid wall mold is extremely difficult.With metal wall molds, the exceptional wall friction causescircumferential splits in the tubing so cast and such splits haveresulted in the exiting tube being pulled out of the bore of the mold asa broken oif length instead of continuously. To correct this defect, oneaspect of the Maxim patent of 1895 (British Pat. 22,708) pertained tothe use of slippery refractory materials such as axially alignedasbestos fibers compacted with plumbago (graphite). Such slippery andrefractory linings greatly increase the workability of solid wallcentrifugal molds for continuous casting; however, these samecentrifugally created frictional forces cause exceptionally high wearrates on such soft materials. Once an annular circumferential depressionhas been Worn into the I.D. of the centrifugal casting mold at thestarting end, a tube is cast having too large a diameter to permitextraction from the exit end. In practice, this is a steady wear processand the bore keeps opening up as the solidified tube is extracted fromthe mold. For the casting of steel, the wear rates can be extremelyrapid and economically disadvantageous.

Due to the foregoing detrimental aspects of solid wall continuouscentrifugal casting molds, a number of nonrotating methods for thecontinuous casting of tube have been conceived. These are invariablybased on the use of concentric inner and outer solid mold walls that arecooled by various means. Such devices are best exemplied by U .S. Pat.2,473,221 issued to Rossi in 1949 (wherein a cast tube is withdrawnvertically downwards from such a mold) and U.S. Pat. 3,022,552 issued toTessman in 1962 (wherein the tube cast between such concentric molds ishorizontally forced out of the casting apparatus by the hydrostaticpressure of the molten metal being cast). Both of these processesutilize non-tapered internal (LD.) molds and as such, are ditiicult tooperate on a continuous basis due to the cast tube shrinking inwardly(thermal contraction) onto the solid mold in the bore. Shrink iits areused to prevent concentrically assembled items from slipping and, in thecase of continuously cast tube, the shrink t can cause complete stoppageor rupture of the cast tube. In order to obviate the foregoing problemthe inner concentric mold has been tapered so that the cast tube movesto a smaller diameter portion of the I.D. mold as it contracts or theconcentric molds are made as short as possible. Such tubing is beingsuccessfully continuously cast by utilizing either tapered or very shortI.D. molds. However, the output rates (withdrawal rates) are fairly slowand must be carefully controlled to prevent either shrinkage binding(too slow a withdrawal) or molten metal seepage (too fast a withdrawal).

The foregoing processes, where used, are of economical value due to theincreased cost of tube and pipe as an end item.

RESUME OF INVENTION The invention pertains to the continuous centrifugalcasting of metallic and non-metallic tube onto the I.D. surface of arotating hollow cylinder which acts as the initial mold of the tubeforming operation. Molten material, to be cast to tube, is continuallyintroduced into the entrance end of the centrifuge and the solidifiedcentrifugally cast tube is continuously extracted from the exit end. Theprocess is greatly enhanced as to casting rates and ease of extractionof the solid cast tube by the techniques of utilizing a vacuum on theinterior of the tube, in its molten and solid state, and/or a positivepressure (above ambient) external to the solidified or partiallysolidified tube at the exit end of the centrifugal casting machine.

THE INVENTION IN GENERAL Analysis of requirements for the continuouscentrifugal casting of tube The inventor has analysed the prior art andthe machanics thereof and has concluded that the extremely lim itedindustrial use of the known processes for the continuous centrifugalcasting of tube (in spite of the manifold economic advantages whichwould certainly accrue to an easily workable process of this nature) isprimarily due to the exceptionally high side-wall forces and attendantfriction created by the centrifugal action. In this respect, referenceis made to FIG. l which shows the volume contraction of a mild steel oncooling from a pouring temperature down to room temperature. As shown,the volume solidification contraction amounts to 7.2 percent.

In a static casting (such as one made in a conventional sand mold), thetotal contraction depends on the solidification contraction and thethermal (solid) contraction. In a centrifugal casting (operating at thehigh G, gravitational, forces necessary to produce a dense casting), thesolidification shrinkage is nonexistant since, as the denser solidgrains grow from the molten matrix of the surrounding liquid steel, theyare centrifuged to the outer surface and form a solid ring of weldedparticles which have already undergone their solidification contractionprior to uniting into a solidified ring. More than this, the thinsolidified ring is in a highly pliable condition at a temperature justbelow its melting point and is readily stretched to its maximumequilibrium diameter under the centrifugal forces involved.Solidification contraction occurs, however; it is evidenced as adecrease in the Wall thickness of the solidifying tube while the outsidediameter remains essentially unchanged. From then on the onlycontraction is the thermal contraction of the solidified ring as itstemperature is lowered, by heat abstraction, from the solidificationtemperature of about l500 C. to room temperature.

By way of demonstrating the side-wall forces involved, and as anillustrative example, we can considered a solidwall mold having al0-inch I.D. and the continuous centrifugal casting of the mild steeltube thereon which has a wall thickness of one inch. The mild steeltube, so cast,

would have a lil-inch O D. and an LD. of 8 inches. Since,

in centrifugal casting the solidification contraction is practicallynil, it can reasonably be expected that, from the point where a solidskin first starts to form on the molten tubes O.D. surface, thesolidifying tube will typically stay in Contact with the mold wall foran axial distance of at least 24 inches. After this point, thermalcontraction of the more solid cast tube wall causes the tube to shrinkaway from the mold wall with attendant alleviation of friction. Thecontact area of tbe steel, from the axial point where it just starts tosolidify to the point 24 inches towards the exit where it shrinks out ofcontact with the mold wall, is 24 inches times the outside circumferenceof the tube or 24 3.14 l0=756 square inches. If the centrifuge isoperated at 50 Gs (a rotational speed whereat one pound of weight wouldactually exert a side-wall force of 50 pounds) the weight of one cubicinch of justsolidifying steel (the normal density of mild steel at thesolidification temperature of 1500n C. is 0.264 lb./cubic inch) wouldhave an effective weight of 50 0.264 or 13.2'

lbs/in.a and this is the force that the cubic inch of steel would exerton a square inch of surface area of the mold wall. Actually, the amountof molten metal pressing on the one square inch of surface area is not afull cubic inch since the volume is that of a truncated wedge as shownin FIG. 2. The inner surface of this truncated wedge is 0.8 sq. inch andthe volume of the wedge is 2 Xl or 0.9

cubic inch. The effective weight of this volume is therefore 0.9 13.2 or11.9 lbs. at the 50G rotational speed involved. Since there are 756 suchvolumes in contact with the mold wall, the total side-wall force is756)( 11.9 lbs. or 9000 lbs. Assuming a coefficient of friction of 0.25,then the force required to pull the tube out of the bore of thecentrifuge would be 0.25 X9000 lbs. or 2,225 lbs. Such a pull-out forcecan create stresses in the cross-section of the solidified steel outerlayer that exceeds the hot tensile strength (which is very low attemperature just below the solidication temperature) of the steel andcause circumferential rupturing of the tube with attendant discontinuouspull-out. Static friction is considerably higher than sliding frictionand the critical period in continuous centrifugal tube casting would beat start-up when the extracting pull is first being applied to the tube.Such circumferential fracturing of the newly cast hot tube is much morelikely to occur at this time.

Solid-wall molds for the continuous centrifugal casting of tube wouldbest be constructed of a material exhibiting the followingcharacteristics. It should be:

(l) Refractory and preferably, have a melting or softening pointconsiderably in excess of the pouring temperature of the molten materialbeing cast to tube. Therefractory requirement is primarily importantwhere the thermal conductivity of the mold wall material is less thanthat of the material being cast.

(2) Highly heat conductive so that the heat of solidification of themolten tube material can be conducted away with sufiicient rapidity toprevent overheating and welding. In this respect, it should be notedthat such highly heat conducting metals as silver, copper, and aluminumcan be utilized as mold walls for the casting of materials havingsolidification temperatures in excess of their melting points. Asexamples, steel has been continuously noncentrifugally cast in watercooled molds of copper and gray iron has been cast (non-continuously) inWater cooled molds of aluminum. Generally, such mold materials arecoated on the contact (with the molten metal to be cast) surface with afairly thin layer of a more refractory and non-galling material such aschromium plate in the case of copper molds and a heavy anodized surfacein the case of aluminum molds. Such high heat conductivity materials canbe used for solid-wall centrifugal casting molds but are not thepreferred ones due to danger of softening at any localized hot spot.

(3) Non-seizing or slippery. Such materials as dense graphite,molybdenum disulfide, or boron nitride exhibit such characteristics.More than this, they are refractory and (with respect to many castingmaterials) are non-wetting as well. Graphite has long been available forsuch use but it has only been in recent times (due to currenttechnological demand) that the denser structural graphites have beenmade available. Even so, dense graphites, molybdenum disulfide, andboron nitride are relatively soft and exhibit rapid wear under theexcessive pullout frictional forces inherent to continuous centrifugalcasting as now practiced.

ICurrent engineering materials such as the dense graphites, molybdenumdisulfide, and boron nitride; the highly refractory metals such astungsten, molybdenum, tantalum, and columbium; and such refractorynonmetallics (in highly impervious form) as -alumina and berylliaexhibit some of the foregoing desirable characteristics and can be usedto advantage as mold materials for the continuous centrifugal casting oftube. The primary drawbacks, however, are the excessive wear rates forthe soft slippery materials and the excessive wall friction of the othermaterials which limits tube extraction.

It is apparent from the foregoing analysis that the sidewall frictionalforces, attendant to such casting machines now known to the art, must bedrastically reduced in order to fully realize the economic advantages ofcontinuous centrifugal tube casting.

Methods by which this can be accomplished are given below as Methods1-5, and hereinafter are referred to by number, as Method l, etc.

Method l.-By use of solid-wall mold materials having low coefficients offriction. This was shown in 1895 by the Maxims of British Pat. 22,708,wherein the use of such slippery materials as axially aligned asbestosfibers and compacted graphite was disclosed.

Method 2.-By centrifugally casting at lower rotational speeds to reducethe G forces and consequently the frictional force. Actually, the Gforce limits for centrifugal casting are well established forconventional prior art methods and are in the general range of from 50to 100 Gs. The 50 G rotational speed is not the lower limit for avoidingraining and sloshing of the molten metal but is considered necessary forthe production of a sound dense casting. Above 100 Gs, thecircumferential stresses produced in the cast tube can causelongitudinal rupture of the tube.

Method 3.-By use of a centrifuged liquid mold as disclosed in the Maxim(Br.) Pat. 22,708.

The foregoing methods are a part of the prior art and can be utilized tothe point where tube can be produced as an end item due to its increasedcost as such.

THE METHOD OF THE INVENTION- SPECIFIC EXAMPLES It is a purpose of myinvention to provide a novel method, using a gas pressure differential,for reducing the side-wall frictional forces to an extent that tube canbe continuously centrifugally cast at such high rates of output that thetube, so produced, can be used, not only as tube, but also, as a -basicstarting item for fabrication into other longitudinal items of structuresuch as plate, angle, railroad rails, I-beam, channel, etc.

Two variants of the novel method herein disclosed are listed as follows:

Method 4.-By creating a vacuum, internal to the tube being cast, so asto partially or completely (depending on the wall thickness of the tube)counterbalance the centrifuged weight of the tube wall by the suction ofthe vacuum.

Method 5.-By raising the atmospheric pressure (exterior to the tube andthe exit orifice of the centrifuge) by a desired amount over that of theambient atmospheric pressure (14.7 p.s.i. is the average standardatmospheric pressure at sea-level) to create a counterbalancing force onthe exterior of the tube to partially or completely counteract thecentrifuged weight of the tube wall.

Method No. 4, the creation of a partial vacuum on the interior of thetube, is the preferred method since it is the more effective means ofaccomplishing the desired counterbalance and introduces other beneficialeffects as well. In my above identified prior patent application, Ser.No. 538,506, the use of an internal vacuum within the centrifugally casttube has been described in conjunction with the continuouscollapse-forming of the tube to longitudinal items of selectedcross-section. In such a process, the tube cavity is sealed at theexiting end by the inward collapse of the tube walls and the weldingtogether of the inner contiguous surfaces of the tube wall. The seal atthe starting (pouring) end of the centrifuge is created by anon-rotating end plate, or disc, the periphery of which is immersed intoan annular trough of a heavy liquid material (such as molten Woodsmetal, lead, or tin).

By continuous heavy suction (via a conduit extending through and sealedto the non-rotating end plate) a moderately high vacuum (an internalpartial gas pressure of a few pounds per square inch) can be maintained.This system exhibits the following advantages, all of which are objectsof the invention:

(a) After an initial purge with an inert gas, at start-up, and thenapplying the suction, the gases given off by the molten metal are of areducing or inert nature (as carbon-monoxide, hydrogen and nitrogen) andthese gases maintain the inner surfaces of the tube in a brightoxidefree condition which permits and facilitates the pressure weldingof the contiguous interior surfaces of the tube one to the other. Itmight be mentioned that these same gases create porosity or blow-holesin ingots cast by the old ingot-mold process and that these gas cavitiesare collapsed to a defect-free solid condition by subsequent rollingwhich welds the clean oxide-free inner sur-faces of the pocketstogether. This is but one of the major advantages from the use of apartial vacuum internal to the tube being cast.

(b) The centrifugal load on the external structural shell of thecentrifuge is greatly decreased since the effective weight of the metalbeing cast to tube is essentially reduced to nearly zero.

(c) The molten metal (being cast to tube) is effectively degassed by theinternal vacuum during its entrance into the centrifuge via a conduitextending through and sealed to the non-rotating seal plate. As a matterof note, it is preferred to vacuum-degas the molten casting metal priorto its introduction into the continuous centrifugal tube casting deviceso as to cut down the amount of gas given off by the molten metal.

(d) The internal partial vacuum materially aids any subsequentcollapse-forming operation.

(e) The internal partial pressure of reducing gases can be maintainedwithin the tube, as will be revealed in the teachings of this invention,for as long as desired and the internal surfaces of the tube will remainbright and oxidefree for subsequent reheating of the tube andcollapsedeformation thereof or, if desired, as a pre-cleaned surface forsubsequent application of an internal oxidationresistant coating ofenamel, plastic, rubber, zinc, tin, lead or the like.

(f) The primary advantage, of course, is the counterbalancing of thecentrifuged effective weight of the molten and solidifying tube wall soas to minimize the side-wall force and attendant friction.

(g) Due to the supporting action of the internal vacuum, greaterrotational speeds (higher than the usual upper limit of Gs) can betolerated and optimum densification of the tube metal can be achieved.Such higher G forces can also be utilized to enhance gravitysegregation(to be discussed later) where so desired. Prior calculations of thisdisclosure have shown that a 10- inch diameter tube having a wallthickness of one inch (8- inch I.D.) wil create an effective side-wallpressure of 11.9 p.s.i. when centrifuged at 50 Gs. By Method 4, I canreduce this side-wall pressure to zero, if desired. Assuming a standardambient pressure of 14.7 p.s.i., a suction of 11.9 p.s.i. interior tothe tube Will just counterbalance the centrifuged Weight of the tubewall. The absolute pressure of the partial vacuum would be 14.711.9 or2.8 p.s.i.

In Method 5, the volume external to the exit end of the centrifuge andthe exiting tube is enclosed, by appropriate means to be disclosed, soas to afford an effective seal which permits the application of a higherthan ambient gas pressure that forces the tube metal inwardly. Thisexternal pressure can be used to counterbalance some or all of theeffective weight of the tube (depending on the wall thickness and the Gforces involved) in the same manner as the internal vacuum of Method 4.

This external pressurization is preferably accomplished with a dry inertgas such as nitrogen, argon, helium, or the like. It is preferred to useMethod 5 in conjunction with Method 4 since, by such a combination,greater tube wall thicknesses or operation at higher G forces can beaccommodated. It should be realized that the illustrative calculationsare based on a mild steel having a density of 0.264 lbs/in.3 at itssolidification temperature of 1500 C. Less dense metals (such asmagnesium, aluminum, titanium, etc.) can be counterbalanced even moreeifectively;

Besides the supporting action, Method 5 has the further advantage ofmaintaining an inert atmosphere which protects the exterior of theexiting tube and the interior mold wall of the centrifuge fromoxidation. Due to this, the refractory metals (such as tungsten,molybdenum, tantalum and columbium) which are prone to catastrophicoxidation, can be used as mold wall materials, provided that theexterior mold surface is jet cooled with suitably reducing fluids thatprevent oxygen attack.

With respect to Methods 4 and 5, it is preferred to utilize higherinternal vacuums (Method 4) and lower external positive pressures(Method 5) where tubes having a smaller diameter and heavier wallthickness are concerned. Conversely, in the production of large diametertubes of thinner wall section, it is preferred to utilize a much lowerinternal vacuum (Method 4) and higher external positive pressures(Method 5) in combination. The

reason for this preference is that the ambient pressure of the air (asstandard 14.7 p.s.i.) creates a back-pressure on the tube which isdirectly proportional to the cross-sectional area of the tube and also,to the pressure differential between the ambient atmospheric pressureand the internal vacuum. As an example, a tube having a l O.D.(cross-sectional area of 78.5 sq. inches) and an internal Vacuum of 4.7p.s.i. (pressure differential of 14.7-4.7=10 p.s.i. with regard to astandard atmospheric pressure) would experience a backward thrust of78.5 in.2 l0 p.s.i. or 785 lbs. In other words, it would require a forceof 785+ lbs. on the tube to counteract the internal suction and pull thetube out of the bore of the centrifugal casting machine. On the otherhand, a large diameter thin-walled tube (30 inches in outside diameteras an example) would have .a cross-sectional area of 709 sq. inches and,if the pressure differential (between the interior vacuum and theambient pressure) was 10 p.s.i., a force of 7090 lbs. would be requiredto get the tube out of the bore of the casting machine. If the 30"diameter tube had a M1 wall thickness and was centrifugally cast at 50Gs, the pressure differential necessary to counterbalance the steelwould be 1A: of 13.2 p.s.i. or 3.3 p.s.i. In this case, the required 3.3p.s.i. could be made up entirely by application of a positive externalpressure (Method of 14.74-33 or 18 p.s.i. and the internal pressure ofthe 30" diameter tube would be 14.7 p.s.i. or the same as the ambientpressure. By this technique, a very small force would be required toextract the tube from the bore of the casting machine since the externalpressure (of Method 5) acts on the periphery of the tube to justcounterbalance the weight of the steel tube at 50 Gs and does not act onon the end (cross-sectional area) of the tube to create a `back-forcewhich must be overcome (as in Method 4) to get the tube out of thecasters bore.

It is readily apparent from the foregoing examples that a very widerange of latitude is available to the operator, in the application of aninternal vacuum (Method 4) and an external positive pressure (Method 5),for ready extraction of a tube from the centrifugal casting machine. Ajudicious (readily calculated) selection of internal and externalpressures is available for all practical casting requirements.

It is preferred to utilize Method 4 and/ or Method 5 to the extent thata slight side-wall pressure exists since, by such contact, more rapidheat extraction takes place. In fact, it is fortunate (in the use ofMethod 4 and/ or Method 5) that solidilication shrinkage of the tube ispractically nil under the centrifugal forces involved since such a lackof shrinkage is necessary for maintenance of side-wall contact. Once asolid outer shell has formed on the tubes outer surface, the thermalcontraction takes place with great ease (only the slight, allowed,uncounterbalanced weight of the tube material exists to oppose suchshrinkage) and a gap then exists between the O.D. surface of the tubeand the I.D. surface of the mold wall. In this 8 non-contact area, heatloss from the tube is due largely to radiation and this is facilitatedby blackening the interior surface of the mold wall in the non-contactarea to absorb the radiated heat.

As an illustrative example (again using a mild steel tube of lil-inchO.D. and a one inch Wall thickness being continuously centrifugally castat 50 Gs). I will allow an uncounterbalanced side-wall pressure of 0.2p.s.i. to exist (the balance of 14.7-0.2 or 14.5 p.s.i. beingcounterbalanced by a combined 14.5 p.s.i. from Method 4 and Method 5, as10 p.s.i., by Method 4 and 4.5 p.s.i. by Method 5). Due to the paucityof stretch in the incipiently formed solid skin, the contact area of themold wall will be less than the axial 24 inches previously assumed andwould now be closer to 16 axial inches. The surface contact area wouldthen be 1rD l6 or 3.l4 l0 l6 or 502 square inches and the total Weightof the steel tube wall in this area would be 502 sq. in. 0.2 p.s.i. (theuncounterbalance weight) or 100 lbs. This weight of 100 lbs. times thecoeicient of friction of 0.25 gives a total pull-out force of 25 lbs.

Such pull-out forces can be made considerably less than those that nowoccur in the conventional (non-centrifugal) continuous casting of solidbillets and, due to this, the extraction rates can be greatly increased.

The length of such a mold (the increased length permits maintenance ofthe exterior pressure of Method 5 over the entire axial length) can beincreased to an extent that an extraction rate of ft./min. is entirelyfeasible. The cross-sectional area of a tube (10 O D. and 8" LD.) is 1r(Z5-16) or 28.3 sq. in. A one inch length of such a tube would weigh28.3 cu. in. 0.264 lb./cu. in. or 7.5 lbs. An extraction rate of 40ft./min. equals 480 in./min. or 28,800 in./hr. and, since one in. oftube length equals 7.5 lbs., the casting rate would be 7.5 lbs./in.28,800 in./hr. or 216,000 lbs/hr. or 108 tons/hr. For the same wallthickness, the casting rate is proportional to the tube diameter and a-in. diameter tube would have a casting rate of 5 108 or 540 tons/hr.Such outputs are entirely feasible if the mold is made sufficiently longor, alternatively, annular gas and liquid bearing cooling rings (to bedisclosed below) are utilized. Where both techniques are used, evenhigher casting rates are entirely feasible providing such a large amountof molten casting material is continuously available.

By use of the foregoing combination of methods, the tube can egress fromthe system at a temperature only Slightly less than its solidicationtemperature (practically no thermal shrinkage whatsoever) and, in thisinstance, the interior of the tube can still be in a semisolid or evenmolten state depending on the rigidity and thickness of the exterior(solid) portion of the tube wall. Cooling external to the exit end ofthe casting machine can then be accomplished with attendant greaterproduction rates or, if desired, decreased length of the centrifugalcasting mold.

Production-wise, such a 50-in. diameter steel tube with one in. thickwalls would be collapse-formed to a two in. thick plate having awidth-of 1rD/2 in. or 78.5 in. or 6%. ft. No process in existence todaycan produce such a product at anywhere near the casting rates noted.

It should be noted that the disclosed process permits the continuouscentrifugal casting of tubes having smaller diameters than are nowfeasible by batch type solid Wall centrifugal casting. In batch-typecentrifugal casting, there are certain limitations as to the length oftube that can be cast for a particular diameter and this is particularlytrue for tubes having small diameters (as less than two inches O.D.).The longitudinal contraction of such tubes, in cooling down from thejust-cast to the extraction temperature, is sufficiently great thatcircumferential rupture will occur if this shrinkage is undulyrestrained. Such restraint is produced by minor ovalness, or out-of-lineof the bore of the centrifuge, end sticking, or surface roughness. Insmall diameter tubes, the diametral shrinkage is insuicient to obviate(shrink away from) such restraining mechanisms and the large amount ofrejections due to such circumferential rupture makes such productionuneconomical. The present process can produce such small diameter tubeson a continuous basis and without ruptures since the only restraint tolongitudinal shrinkage would be the side-wall friction due to theallowed un-counterbalanced tube wall weight which can be madepractically nil.

kIn the case where the pressure differential of Method 4 and/ or 5 issufficiently great to more than just counterbalance the centrifugalweight of the metal being cast, it might be expected that the tube woulddecrease in diameter to the extent that it would lift away from the moldand permit ingress of air or inert gas into the vacuum of the tubesinterior via bubbling through the molten zone of the tube. This can anddoes happen, but not suddenly beyond the point where the pressuredifferential overbalances the zero-point.

A stable-state range exists for pressure differentials in excess of thezero-point and this is due to the surface tension of the molten materialbeing so cast. This operating area (pressure differential beyond thezero-point) is not actually used since the stable-state condition is notthat broad and can readily be destroyed by any out-of-balance or othervibration producing condition of the rotating system. It does, however,afford a usable margin of safety for the condition of exactcounterbalance.

It is one of the important features of this inventions disclosure toutilize the advantageous system of a vacuum internal to the tube beingcast (Method 4) in the instance wherein tube itself is the end-iteminstead of a longitudinal structure formed by inwardly collapsing thetube walls over its entire out-put length. In the practice of makingtube for its own use, a tube (having a capped or crimped Vacuum sealedexit end) is used as the starting tube so that the desired vacuum(depending on the wall thickness of the tube, the density of the moltentube material, the G force of the centrifuge, and the ambient pressureof the atmosphere) can be drawn on the tube interior. The machine thencontinuously produces a long length of solidified rotating tube whichexits into an axially aligned cradle which permits such combined egressand rotation. Such a cradle can rotate with the tube by virtue of thesame device mechanism as that which rotates the centrifugal castingmachine and a multiplicity of axially aligned rollers supports theperiphery of the tube and, at the same time, can either permit or causethe tube to move axially away from the casting machine. In the casewhere axial movement is permitted, the rollers are mere idlers which areattached to and rotate with the cradle. In the case where they cause thetube to move axially, the rollers are spring or piston loaded onto theouter surface of the tube to give a friction drive contact which pullsthe tube from the bore of the centrifuge as is necessary where aninternal vacuum (Method 4) which causes a suction, must be opposed. Therollers, in this instance, are suitably driven by sun gears (via asuitable gear cluster system for such power transmission) and areactivated or de-activated by a suitable clutch mechanism. Suchmechanisms are well known to those practiced in the art of rotarycoupling and un-coupling. At the same time, there is an axial gap inthis cradle system, near the exit end of the centrifuge, withappropriate torch re-heating means and rotating opposed swaging orforging hammers which move in axial synchronization with the exitingtube and swage or pinch a re-heated section of the tube to aVacuum-tight closure after any desired length has been produced. Thepinch or swage closing mechanism then returns to the initial startingplace where its operation is re-commenced after another appropriatelength of vacuum-sealed tube has been produced. Along with the swagingmechanism, and axially further away from the centrifugal caster by anyappropriate length (a two foot long swaged section and a 200 foot lengthof tube between swages would limit the loss of tube due to swaging toone percent), is located an appropriate cut-off device which travels inaxial synchronism with the exiting tube and severs the tube at themiddle of the swaged or forgeddown closure so as not to destroy theintegrity of the internal vacuum. After cutting the tube in the axialcenter of the swaged section, the cut-off returns to its starting pointfor recoupling to the axial travel mechanism and cut-off of the tubesection at the appropriate time. By this synchronized and discretelyrepeatable sequence of swaging down and cutting olf the exiting tube,the integrity of the internal vacuum (with its manifold advantages) ismaintained during and after the tube casting operation.

It is convenient to forge-atten the exiting tube (just as a soda-strawcan be pinch-flattened in a selected area between thumb and foreiinger)at the separation point. However, even though this serves as a simplemeans of sealing and maintaining the integrity of the internal vacuum,it is the preferred method of this invention to swage or peripherallyhammer-forge such separation points to a solid round having itsforge-welded center-line coincident -with the axis of the tube. Theseend closures (after separation of the tube lengths at the mid-lengths ofthe solid swaged-down closure) can be cut from the tube ends with anintegral portion of the tube length as long as desired. Such cut-offclosure lengths are conveniently used to fabricate pressure bottles ortanks for Oxy-acetylene, propane storage and the like. In this manner,the closure part of the tube is not subject to re-melt but affords greateconomies in the manufacture of pressure tanks and storage vehicles.

My preferred means of extracting (pulling the tube out of the bore ofthe caster in opposition to the suction of the internal vacuum) is topower the rotating swaging apparatus so that, once it has swaged downthe tube to a vacuum-tight solid round, the swaging apparatus remainsgripped to the solid reduced tube closure and pulls the tube out of thebore. The axial travel of the apparatus can be powered by any convenientmeans (such as a chain drive, cog-wheel, worm screw, etc.) and can begeared to or be separate from the rotational means as desired. Thesystem utilizes two such swaging-down and pull-out mechanisms so that,while one mechanism is pulling out the tube, the second mechanism can beswaging down a tube closure sorne 200 `feet closer to the centrifugalcaster. Once the second mechanism has swaged down and gripped the tubeclosure for powered pull-out, the iirst mechanism (axially lfarther awayfrom the centrifugal caster) then severs the tube lengths from eachother at the midlength of the swaged-down closure so as not to destroythe vacuum seal. The first mechanism is then returned to the startingpoint to restart as the second mechanism. The two mechanisms thuscontinually replace each other at the starting point. Alternately, byway of decreasing the axial floor-space requirements, the swage-down andpull-out mechanism can grip the swaged-down end of the tube being pulledout and, at the same time, sever the completed length which then isreleased from the accordion-pleat cradle (a series of idler supportswhich pull out at regular intervals to support and align the rotatingtube sections between the swage-down mechanisms) and rolled off at rightangles for storage or processing. This is not the preferred means sincea grip slippage would result in the tube being sucked back into the boreof the caster with attendant destruction of the internal vacuum,increase in the side-wall friction and stoppage of out-put for repairs.In the preferred means (using some length,

as the 200 foot example, and more floor-space, depending on the tubelengths produced), any slippage of the grip merely brings the pull-outmechanism into Contact with the belled-down part of the tube and createsa positive and safe pull-out.

In the foregoing manner, long sections of tube (like straightsausage-links) are produced which have an internal vacuum of a partialnature. The internal surfaces of these tube lengths are clean and bright(due to the inert or reducing nature of the gases contained therein) andthis permits the collapse deformation thereof to longitudinal structure(at an appropriate reheat temperature) with roll-Welding of the cleancontiguous interior surfaces. The partial interior vacuum, along withthe clean bright interior surfaces, is very effective in promoting theapplication of interior coatings to the tube since (by clipping the tubeend while immersed in the iiuid coating medium and replugging theopening once the exact amount of coating has been sucked into theinterior of the tube-length) the tube can then be rotated-in-place toevenly coat the tubes interior surface while the coating is beingheat-cured, catalytically cured or solidified in place as suits itsnature (whether of or ganic, non-organic or metallic). The cleaninterior surfaces accept such coatings with excellent adhesion.

In the collapse-deformation and roll-welding of such tube, the tubesection can be collapse-formed partially (over its entire length) orcompletely collapse-deformed (over a part of its length), withappropriate preheating, so that a positive internal pressure (aboveambient) is built up inside the tube. The back end of the tube is thenperforated to permit escape of the internal gases for continued hotcollapse-deformation and sizing to a completed item of longitudinalstructure. In this manner, the internal vacuum does not suck in moistair which could contaminate the bright-clean interior surfaces to thedetriment of their being roll-welded together.

The long lengths of tube (they can readily be made as mile-long lengthsby exiting the tube onto a body of Water, such as a bay or down a streamor river, which tloats the tube and acts as the support cradle), havingan internal vacuum as a result of both ends being swaged closed, canthen be cut up into desired lengths for use (or for sizing and/or grainrefinement since the ends are appropriately capped) or they can remainunchanged for oat-shipment to any desired shore-line location on earthby bundling into appropriate rafts. Such lengths can then be extendedinland (by means of bag rollers and use of the already laid pipe orpipes as a rail-line) for end cutoi and Weld or other attachment asmile-long lengths. The savings in transportation costs and decreasedwelding for pipeline fabrication is readily apparent.

Whereas the example calculations of this disclosure have been based on amild steel tube product, it should be realized that this is by way of anillustration of convenience. The innovations herein disclosed areapplicable to a Wide range of tube materials of a castable anddeformable, when hot, nature and include such organic materials asplastics and rubbers; inorganic materials such as glasses, and most ofthe industrial metals now used, the only criterion being that thematerials be meltable (either exterior to the casting machine or aselectrodes or powders which can be conveniently melted by arc methodsinternal to the machine without degradation due to the partial vacuum ofinert atmosphere thereinsuch metals conveniently electric or plasma arcmelted internal to the system would be the highly reactive ones such astitanium, zirconium, beryllium, etc.) and, subsequently, deformable.

The mold Walls of the centrifuge can be conveniently made of slipperyand generally non-wettable materials such as boron nitride, graphite,molybdenum disulfide, etc. which (under the conditions of minimizedside-wall pressure resulting from application of Methods 4 and/0r 5 tothe system) exhibit entirely satisfactory surface life with minimizedwear. Due also to this minimized sidewall pressure, the mold walls canbe made of various metals (Without attendant danger of welding orseizing between the mold Wall and the tube material being cast). Also,due to the greatly reduced loads of the mold walls resulting from thecounterbalancing action of Method 4, the .mold Walls can oftentimes beconveniently made of such refractory inorganic materials, having goodheat conductivity, as alumina and beryllia. The range of castable tubematerials and mold wall struc- 12 tural materials is exceptionally broadwhen used in conjunction with the methods herein disclosed.

My continuous centrifugal casting process not only produces a Wide rangeof metallic and non-metallic tubular products for use as such but itproduces this variety of tube at such high rates of output (e.g.,hundreds of tons per hour) that the tube can be economically and veryadvantageously used as a basic item for the production of other items oflongitudinal structure. lt is therefore a continuous casting processthat is highly competitive when compared to the current continuouscasting of solid billets and slabs. More than this, the collapsedeformation of such continuously cast tube (as a basic starting item ofproduction) into other longitudinal structural shapes can be readily andmuch more economically done than by current techniques and this can beaccomplished by the use of very light mills (as light rolling mills) andwith very few passes. Capital investment is thus greatly reduced andthus augments the other economies of the process.

OBJECTS OF THE INVENTION lt is an object of this invention to reduce theside-wall forces and attendant friction of solid-Wall continuouscentrifugal tube casting machines by application thereto of a vacuuminternal to the tube being so cast (Method 4) and/ or a positivepressure, external to the exit orifice of the caster and the tube O.D.(Method 5).

Another object of this invention is to utilize a vacuum seal at theentrance or starting end of the centrifugal cast ing machine for thepurpose of reducing the pressure internal to the tube being cast.

Another object of the invention is to continuously collapse the tube toa longitudinal structural solid shape so as to form a vacuum-tight sealfor the tube at the exiting end.

Still another object of the invention is to collapse a limited portionor section of the tube, as it exits from the casting machine, to formvacuum tight closures at specified intervals along the length of thetube.

Another object is to cut ott such lengths of tube at the mid-length ofthe closure (limited collapsed section) so as to maintain the integrityof the vacuum internal to the tube and to obtain long useable lengths oftube having such closures at both ends thereof.

A still further object is to produce and maintain a vacuum internal tothe tube being cast (Method 4) as a means of reducing the side-wallforces and attendant friction as well as for other advantageous reasons.

Another object is to produce and maintain a positive pressure of aninert or reducing gas external to the exit orifice of the caster and thetube 4O D. (Method 5) as a means of reducing the side-wall forces andattendant friction as Well as for other advantageous reasons.

Another object is to utilize an extended-hot-zone at the starting end ofthe caster in order to accentuate the effects of gravity segregation toobtain a useful result such as a lower carbon surface on steel sheet foruse in the automotive industry. Normally, an unextended hot zone is usedfor layering of the material being cast to tube in the teachings of thisinvention.

Another object of this invention is to utilize an annular section ofpyrolytic material (such as pyrolytic graphite, pyrolytic boron nitride,graphfoil, etc.) in the hot zone or extended hot zone section of thecaster in such a manser that the c direction (the direction of highthermal insulation) is perpendicular to the axis of the centrifuge andthe a direction (the direction of high thermal conductivity) is parallelto the axis of the centrifuge in order to enhance and equalize the heatdistribution in that area.

Another object of the invention is to utilize annular irises of carbonor other refractory materials at the exit end of the centrifuge as meansof sealing the enclosure of Method 5.

An additive object is to utilize a multiplicity of small radial holespenetrating such annular seal rings whereby either an inert or reducinggas or a stream of liquid can be forced therethrough onto the rotatingsurfaces to act as a gas or liquid bearing (under high pressure) withattendant cooling and sealing action.

A still further object of this invention is to so increase the castingrate of continuous centrifugal tube casting machines, utilizing asolid-wall mold, that the tube product can be used as a basiccontinuously cast item for economical conversion into other items ofstructure on a continuous or non-continuous basis.

The novel features which are considered characteristic of this inventionare set forth with particularly in the appended claims. The inventionitself both as to its organization and method of operation, as well asadditional objects and advantages thereof, will best be understood fromthe foregoing description when read in connection with the accompanyingdrawings, in which:

FIG. 1 is a graphical representation of the change in specic volume of asolidifying and cooling steel;

FIG. 2 is a geometrical diagram of a truncated wedge sectionhypothetically removed from the cast tube for illustrative purposes;

FIG. 3 is a partial axial sectional view of a horizontal centrifugalsolid-wall continuous tube casting machine with seal means at theentrance and exit ends thereof;

FIG. 4 is an axial sectional view of an embodiment of the exit end of asolid-wall centrifugal tube casting machine which depicts means ofenclosure thereat to effect a positive pressure (above ambient) externalto the exiting tube as per Method 5;

FIG. 5 is a partial axial sectional View of a vertical centrifugalsolid-wall continuous tube casting machine with seal means at theentrance end thereof;

FIG. 6 is a partial axial sectional view of a plasma torch arrangement,utilizing a bellows vacuum seal, in the retracted position;

FIG. 6A is a similar view of the plasma arrangement in the extendedposition.

.DETAILED DESCRIPTION-THE PROCESS Referring now lto the drawing indetail, and in particular to FIG. 1 (redrawn from Wullfs Metallurgy forlEngineers) it can be seen that a centrifugally cast mild steel tubewill experience a volume shrinkage of about 6% or a diametricalshrinkage of 2% in cooling from the soliditication temperature of about1500C. to a tempera- -ture of about 330 C. under centrifugal castingconditions. It can also be derived that the diametrical shrinkage of acentrifugally cast mild steel tube in cooling from l500 C. down to 700C. is about 1.5%. The specific volume contraction curve of FIG. 1,illustrates the amount of shrinkage attendant to the cooling of a mildsteel casting under normal or static conditions and is reproduced hereinfor information purposes.

FIG. 2 is illustrative of a solid geometrical configuration wherein asquare inch area on the periphery of a -in. diameter tube, having al-in. wall thickness, is radially projected inwardly onto the axis ofthe tube to form a truncated wedge within the contines of the radialprojection lines and the exterior and interior surfaces of the tubewall. The projection of the l-in. sq. area on the exterior surface ofthe tube onto the tubes axis cuts out a rectangular area on the interiorsurface of the tube that is l-in. long and has a circular length of0.8-in. on the adjacent side. The inner rectangle has an area of 0.8X1or 0.8 sq. in. The volume of the truncated wedge is, therefore, 0.9 cu.in. and this volume bears on the 1 sq. in. of exterior tube surfaceunder the influence of the centrifugal action. The geometricalconfiguration is used to illustrate the decrease of volume bearing onthe O.D. surface of a tube as the diameter becomes smaller and thecorresponding decrease in bearing pressure (p.s.i.).

In the following drawings pertaining to continuous centrifugal castingmachines and devices, such means as cooling of the mold, tube withdrawaltechniques, trunnions, bearings, rotational mechanisms, and the likewhich are well known to the prior art, are not shown and have beenomitted for the sake of brevity.

Reference is now made to FIG. 3 which is an axial cross-sectional viewof a horizontal solid-wall continuous centrifugal casting machine whichrotates about its axis 1. The molten material 2 to be cast to tube, iscontinuously introduced into the entrance end 3 via the conduit 4 andpours into the annular distributing trough 5 of the refractory part 6.The refractory part 6 is encased in a structural metal housing 7 whichextends towards the exit end 8 of the centrifuge as the solid mold wall9 the exterior surface of which may be cooled by a multiplicity ofperipherally spaced jets of cooling liquid (not shown). The moltenmaterial 2 overflows the ledge 10 which is lined with an annular ring11, of axially aligned pyrolytic material for rapid axial heatconduction and radial insulation and constitutes a hot zone 16 and formsan axially flowing ring of molten material 12 which freezes to a solidtube 13 by heat conduction to the mold wall 9 in area 14 and byradiation to the blackened mold wall interior in area 15. At theentrance end 3 of the centrifuge and axially external to the refractorypart 6 is an annular trough 20 which is partially filled with acentrifuged heavy liquid 21 of a high boiling nature (as Woods metal,molten tin or lead, etc.). A non-rotating end plate (disc) 22 has itsouter periphery 23 immersed in the annular trough uid 21 and constitutesa vacuum seal for the casting machine at its entrance end 3. The endseal disc 22 has circumferential gutters 24 which collect any cascadingfluid 21 and return it to the trough 20 at the bottom side. The moltenmaterial conduit 4 as well as an inert gas purge tube 25 and a vacuumsuction line 26 extend through the end plate 22 and are attached theretoby leakproof seals. By means of the purge tube 25, the cavity 30 of thetubes interior is purged with an inert gas and a vacuum is then drawn onthe interior cavity 30 via vacuum tube 26. The diameter of the trough 20is considerably greater than the diameter of the centrifuge and thediameter of the liquid level of the sealing material is also quitelarge. By this means, greater access area (via sealed but removable portholes) is available in the end plate 22 for insertion of requiredmechanisms such as plasma torches, rotary skimming devices, etc. asneeded. The trough 20 is deep enough to contain all of the seal fluid21, without overow, when rotation is stopped.

Exterior to the exit end of the centrifugal casting machine may be a setof opposed forgin rolls 34 and 35 which travel axially and insynchronismwith existing tube 13. At the same axial location and atright angles to the plane between the axis of the forging rolls (34 and3S) may be two opposed banks of burners such as plasma torches (notshown) which maintain the heat of the exiting tube 13, or bring it to adesired forge-welding temperature. These forging rolls 34 and 35 movesynchronously and axially along with the hot tube and gradually cometogether with sufficient force to collapse a small portion of the tube(as a 2 ft. length) to a solid round having a forge welded interiorjoint 36 which is vacuumtight. Such collapsed sections of the tube canbe as far apart as desired (e.g. every 300 ft. of tube length) andcomprise the vacuum seal to the tube at the exit end of the centrifugalcaster. Further on, and after another seal has been so forge-closed, thesolid section 36 can be cut off at its mid-length for removal of thediscrete length of vacuum sealed sausage-like tube lengths, for use aspreviously described. It can be appreciated that other conventionalmeans, such as swaging. flat-crimping, etc. can be used to form thediscrete collapsed section for vacuum closure at point 37 of the hottube. Also, axial travel of the sealing rolls (34 and 35) can beextended (as to 300 plus feet) so that they act as powered pullout gripsfor the tube so cast. The tube can also be sealed at the exit end bycontinuous collapse-deformation thereof to longitudinal items ofstructure in accordance with the teachings of my prior patentapplication Ser. No. 538,506.

By means of the forged tube closure 36 and the end plate seal 22, avacuum can be drawn (via conduit 26) on the tube cavity to an extentthat it partially or completely counterbalances the side-wall force andresulting friction of the tube being cast in accordance with Method 4.

It should be noted that the hot zone 11 can be lengthened beyond thatnecessary for ring layering 12 so as to create an extended-hot zone 16so that slow cooling of the molten tube can be accomplished. In thismanner, when desired, accentuated gravity segregation results (eg. deltaferrite being centrifuged towards the outside surface of a mild steeltube which is later to be converted to automotive sheet steel).

FIG. 4 is an axial sectional view of the exit end 8 of anotherhorizontal solid mold centrifugal tube caster according to the inventionand is illustrative of the annular exit end closure 40 utilized in theapplication of Method 5.

In FIG. 4, the collapsing and forge Welding rolls 34 and 35 have alreadybeen described as a means for sealing the tube 13. Whereas the tube 13and the mold 9 are rotating, the annular end closure is stationary. Aninert or reducing gas 41 (inert is a relative term since a gas such ascarbon dioxide, which is oxidizing to hot steel, is practically inert tohot aluminum and can be so used in the casting of aluminum tube) isintroduced into the end closure 40 via the high pressure gas tube 42 andthe pressurized gas 41 acts on the outer surface of the tube (bothexterior to the exit end and in the shrinkage gap between the tube andthe mold wall of the centrifuge) and supports it (counteracts thecentrifugal weight of the tube wall) to a desired extent. The endclosure 46 is sealed at the annular area 43 (on the OD. of thecentrifugal caster at its exit end 8) by means of an iris ring 45 ofcarbon, graphite, or boron nitride leaves 46 which overlap each other(as camera iris blades do) to form an annular ring 45 of such blocks(leaves) in friction contact with the 0.D. of the mold wall at area 43.The iris ring 45 is contained within an annular groove 47, the openingof which faces inwardly, and this groove encompasses a pressure chamber48 (radially exterior to the iris ring 45) which is pressurized by aninert gas 49 introduced via conduit 50. A multiplicity of iris leaves 46make up the iris ring 45 and these leaves are each attached at one endto the groove 47 by means of pivot pins 51.

At the other side of the pressurized enclosure 40 and at area 53 on theO.D. of the tube 13 is'another similar iris ring 55 which seals theenclosure 4t) at the surface of the exiting tube 13. Such iris rings asdescribed, are not the preferred means of sealing the enclosure 40 sincethey exert a considerable wiping force and wear at a fairly high rate.The preferred means is to utilize an annular iris seal ring 45A which ismuch the same as that of 45 except for a multiplicity of small radialholes 56 which exist over the entire iris ring 45A and conduct a highpressure inert gas 49A onto the outer surface of the mold wall 9 at area43A. In this manner, the iris ring 45A acts as a gas bearing and doesnot actually contact the rotating surface of the mold. Due to this, Wearof the iris ring 45A face areas is eliminated and the escaping gas ofthe bearing face maintains the desired pressure of the enclosure 40. Theiris ring 55A which seals against the rotating tubes O.D. surface atarea 53A can also utilize the gas bearing technique; however, it issometimes preferred to use a liquid bearing at the area 53A for thefollowing listed purposes:

(1) A heat extractive coolant of a non-oxidizing nature (as a mixture ofwater and methyl alcohol). Such liquid bearings can also be used to coolthe exterior surfaces of the mold wall 9,

(2) As a quenchant (as a brine plus a suitable reductant) for thepurpose of hardening the tube for use as heat-treated pipe. In thisinstance, the forge welded closures at point 37 would be normalized andsubsequently removed from the pipe. The balance of the quench hardenedpipe would be tempered to a desired hardness and strength level prior toremoval of the forge welded ends and breaking of the internal vacuum.

3. A liquid bearing of lead, tin, zinc, aluminum, etc., or desiredalloys thereof (as lead-tin) would be used (in the molten state where anexterior coating of such metals is desired for corrosion protection ofthe pipe. Coincidentally, a steel tube could be heat-treated byaustempering with such molten metal liquid bearings.

Where liquid or gas bearing irises 43A or 53A are used for sealing theend closure 40 and centering the exiting tube 13, the iris blocks(leaves) 45A and 55A can be made of other materials such as copper,steel, alumina, or other non-metallic materials, etc. since they do nothave a frictional contact with the outer surfaces of the mold 9 or thetube 13.

FIG. 5 is a representation of a starting end 3 vacuum end seal for avertical continuous centrifugal tube casting machine (as of the typedepicted in British Pat. 984,053 and elsewhere and is presented as apartial axial crosssectional View.

In FIG. 5, the axis 1 of the centrifuge is vertical and the moltenmaterial, to be cast to tube 13, is introduced into an annulardistributing trough 5 via conduit 4. The molten material 2 is sluicedhorizontally so that its direction of flow has tangential coincidencewith the rotational motion of the molten material 12 in the distributingtrough 5. The cavity of the molten and solidied tube 13 contains apartial vacuum (Method 4) of inert gas by virtue of being sealed beyondits exit end (not shown) by the inward collapse and forge welding of adiscrete section of the exiting tube 13 and, at its entrance end 3, by anon-rotating seal plate (dish) 22 which has its periphery 23 immersed ina dense high-boiling liquid 21 (as Woods metal, cadmium, lead, tin andalloys thereof) contained in an annular trough 20. The convex side ofthe seal plate (dish) 22 has an annular gutter 24 which inhibits accessof air to the molten metal 21 of the seal and, also, prevents anyinadvertent escape of iiuid from the trough. Under non-rotating (stopperiods) conditions, the liquid levels of the fluid 21 are as shown bydotted lines 27 while, under the centrifugal forces of casting, theliquid levels of the uid 21 assume the positions shown by the verticallines 28. It can be appreciated that the periphery 23 of the dished endplate 22 is always immersed in the uid 21, whether the centrifugalcaster is operating or not, to form an effective vacuum seal. Suchorifices in the end plate 22 as the purge tube 25 and the suction tube26 are the same as in FIG. 3 and the other numbered points (notdiscussed herein) are the same as in FIG. 3 except for the verticalattitude. The molten material 2 enters the conduit 4 by way of aconventional trap 29 as a means of maintaining the vacuum Within theinternal cavity 30.

The central area of the end plate 22 is reserved for other entranceports as required in such a system (as the vertical water-cooled shaft,which is an extension of the rotary mandrel used in the verticalcentrifugal tube casting machine disclosed in (British Pat. 984,053; or,the bellows encased plasma torch of the following FIGS. 6 and 6A).

The exit end 8 end closure 40 as shown in FIG. 4 is also used in theapplication of Method 5 to the vertical system although not shown sinceit would vary but slightly from that already disclosed.

It can be realized that the straight sausage-like links of vacuum sealedtube, or item of longitudinal structure formed by the continuouscollapse of such exiting tube,

would have to be of fairly limited length due to height restrictions.Due to height restrictions, the vertical sys- 17 tems are not preferredover the horizontal continuous centrifugal tube casting machines hereindisclosed.

The vacuum seal means of FIG. can also be used in conventional,non-centrifugal, vertical continuous casting of solid -billets and canbe used as means of applying Method 4 (a vacuum above the pool of moltenmetal beting cast to billet) to this older continuous casting means. Byapplication of such a vacuum, the hydrostatic forces on the moldside-wall can be reduced and the extraction rate speeded up.

The .great advantage of the continuous centrifugal casting processdisclosed herein concerns the rapid continuous casting of thinnertubular walls of high density material that has optimum integrity with aminimum of cross-sectional reduction by subsequent working (as rollingto structure).

FIG. 6 is a partial axial sectional View of a retractable plasma torch71 confined within a vacuum sealing bellows 72 and located at the area70 of the end plate seal 22. The purpose of the torch or torches is topreheat the refractory part 6 (of FIGS. 3 and 5) prior to start-up ofthe tube casting machine; and the bellows 72 is merely a means ofmaintaining the vacuum within the tube cavity 30 during extension foruse and subsequent withdrawal (as shown in FIG. 6) out of the hot areaof the cavity 30.

In FIGS. 6, the annular ange 73 seats against the orifice lip 74 of theend plate 22 and acts as a heat-shield to prevent overheating of thebellows 72.

FIG. 6A shows the plasma torch 71 in the extended or use position and,in this case, the annular fiange 7S seats against the orifice lip 74 andacts as the heat barrier. The plasma torch is encased in a refractorymaterial 76 such as alumina.

Whereas the mechanism is shown in connection with the vertical castingsystem, it is also applicable to the horizontal systems of thisdisclosure.

Stopping and starting procedure In stopping the machine, the axialtravel of the tube extraction device (as gripped forging rolls 34 and 35of FIGS. 3 and 4) is stopped by a suitable clutch mechanism (not shown)and the tube is allowed to rotate along with the centrifugal caster.Coincidental with the extraction stoppage, the input of molten material2 is terminated and the tube 13 is allowed to solidify within the boreof the casting machine. Normally, the machine is kept rotating in anynormal interval between stopping and starting of tube casting. Once thetube extraction is stopped and the tube has solidified overall(including the heavy material section filling the annular trough 5), thepositive pressure of inert gas (from the exit end 8 enclosure 40) willseep into the tube cavity 30 (as in FIGS. 3, 4) via the crevice betweenthe contracted tube wall O D. and the mold wall I.D. Alternately andpreferably, once the seepage begins, the vacuum can be gradually brokenby an inert gas purge via purge tube 25, the suction via tube 26 beingstopped.

In starting up, from a normal rotating interim stop, the plasma torch 71is inserted into the cavity 30= to its predetermined full extension andturned on so that its flame melts the solidified tube material in theannular trough 5. Once this has been accomplished, a desired vacuum isdrawn on the cavity 30, a desired positive pressure is created in theend closure 40, the axial extraction is recommenced, and the appropriateamount of molten material 2 is continually introduced to the system. Theplasma torch is then turned off and withdrawn as shown in FIG. 6.

If the centrifugal casting machine requires repairs in `areas notcovered by the tube 13, the rotation of the centrifuge may be stoppedonce the tube material within the bore of the centrifuge has completelysolidified. After repairs have been made the start-up sequence is aspreviously noted.

In the instance Iwhere repairs or replacements have to be made to themold 9 or the components of the refractory part `6, the cast tube isallowed to completely solidify in the bore of the machine whilerotating, but without extracting the tube or adding molten material 2.See FIGS. 5-6a. Once solidification has been completed and the vacuumbroken, the plasma torch 71 is inserted into the cavity 30 and turned onto quickly remelt the surface material 2 in the annular trough 5 and thetorch is then turned off and withdrawn. The tube extracting mechanism isthen brought into action 'and solidified tube is pulled out of the boreof the casting machine for subsequent use as a starter-blank. Once thetube blank is clear of the bore of the machine, rotation is stopped andthe necessary repairs are made.

On start-up, the cast starting-blank is moved back into the bore of thecaster (by any suitable reversing mechanism), an inert gas purge is madein the cavity 30, rotation is started up, the material in the trough 5and part of the inserted end of the starting-blank is melted down withthe plasma-torch, a vacuum is drawn on the cavity, a positive inert gaspressure is created in the enclosure 40, the plasma-torch is shut offand withdrawn, molten material 2 is continuously introduced via spout 4and extraction is simultaneously commenced.

Grain refinement In general, centrifugally cast metal tube ischaracterized by columnar grains extending radially inwards from theexterior surface. Such grain type is an advantage where the tube is usedat elevated temperatures and pressures since a coarse-grained structureinhibits creep deformation. However, for most purposes, a fine grainedmaterial is desired due to its more favorable mechanical properties.Where the tube is collapsed and roll-sized to structure, such grainrefinement can be accomplished due to the hot-working recrystallization.In the instance where the tube is to be used, as such (as for oil linepipe, etc.), grain refinement can be -accomplished either during thecontinuous centrifugal casting process or subsequent to its cooling toroom temperature.

In the first instance (grain refinement coincident with tube casting), ashearing action can be set-up between the external shell of alreadysolidified metal and the interior layer of still molten metal (as inarea 14 of FIG. 3). This can be done by mechanical or magnetic means andthe layer of still molten metal can be either sloweddown or speeded-uprotationally so that the still molten metal has a circumferential speedthat is different from that of the already solidified exterior shellmetal. In this manner, the shearing action at the solid-liquid interfacedestroys the columnar grain growth and creates an equiaxed fine grainedstructure in the solid tube metal.

Such differential rotational speed between the solid exterior shell andthe inner still molten layer of metal can be caused by an interiorrefractory drum (of light, hollow construction and having an O.D. whichis lessthan the I D. of the molten metal wall 12) which rotates eitherfaster or slower than the centrifuge and is driven by a cooled shaftextending through the stationary end seal plate 22. Such differentialsolid-liquid interface shear can also be created by a rotating magneticflux internal to the centrifuged tube by an adaption of the method ofPestel as disclosed in U.S. Pat. 2,963,758 of 1960 when metal tube isbeing produced.

Grain refinement of the tube metal once it has exited from the castingmachine can be accomplished by pulling the hot exiting tube through arotating sizing bell or by drawing the tube, in the cold state, throughnon-rotating internal and/or external sizing dies which cold-work thetube metal while sizing it. Where discrete lengths of tube, having theends sealed by forged closures, are made, a high pressure aperture canbe made in one end and the tube length can be hydroforged as taught inU.S. Pat. 2,931,744. In both cases, where cold working is done on thetube metal, grain refinement is accomplished by subsequent reheating toits recrystallization temperature.

Gravity segregation One of the limittaions encountered in centrifugalcasting concerns the centrifuging of denser constituents towards theoutside surface (and, conversely, lighter constituents towards theinterior surface) by the high G centrifugal forces. Under normal, fairlyrapid, solidification this is no problem but it is sufficiently severein some alloy systems as to obviate or limit the use of centrifugalcasting. The variation of composition from the interior to the exteriorsurface of a centrifugal casting is termed gravity segregation and hasbeen considered as either a limitation or a nuisance by centrifugalcasters.

It is a purpose of this invention, and one of the teachings disclosedherein, to enhance `and utilize gravity segregation to a useful purpose.

yThe specific method of accomplishing or enhancing gravity segregationto effect a useful purpose is to introduce an extended-hot-zone at thestarting end of the continuous centrifugal casting system hereindisclosed. The Maxim process has a hot zone at the starting end of thecaster for the purpose of preventing a knobby surface (to enhance theleveling or smoothing action) and another invention, U.S. Pat. 2,754,559issued to Fromson in 1956, utilizes an initial hot zone to enhancelayering or smooth spreading out of the molten metal to be solidified ontop of a flat liquid mold of lead. In the process disclosed herein, thehot zone is appreciably extended (where desired to enhance gravitysegregation and only in this instance is the hot zone so extended beyondthat required for effective leveling or layering of the molten steel) sothat segregation Iwill be emphasized and can be utilized usefully aswill be explained in detail later on.

Automotive sheet steel (used for the exterior body covering), isnormally made from rimmed-steel ingots even though it would beconsiderably cheaper, if the desired properties were present, to utilizecontinuously cast slabs or billets instead of remaining with the oldingot process. The reason for this is that rimming-steel exhibits avigorous boiling action on pouring into the ingot mold and this createsa scrubbing action at the solidifying surface of the ingot. The resultis that rimmedsteel ingots have a fine grained exterior layer of fairlylow carbon content. When such ingots are rolled, the surface of thesheet is smoother and takes a better polish than steel made by otherprocesses. It also has better deep drawing qualities. The spattering(which creates a rim on the ingot mold and is the basis for the termrimmed-steel) caused by the release of gases, with resultant vigorousboiling action, is the main reason that rimmed steel cannot beeffectively cast by current continuous casting processes.

Rimrning-steel can be cast in the centrifugal process using a moldhaving a fairly large diameter (as 3 feet) since any spattering merelyends up on the opposite interior surface of the tube and is not oxidizeddue to the internal inert vacuum. The scrubbing action is absent,however, since the released gases are directed inwardly by thecentrifugal forces. Centrifugally cast steel does, however, have therequired density since it is pressure cast under optimum conditions.

I-f, however, an extended-hot-zone is used, either with rimming steel orwith semior fully-killed low carbon steel, the delta ferrite(essentially pure iron) solidifies first and, being solid and denserthan the balance of the molten metal, centrifuges to the exteriorsurface. The resultant centrifugally cast tube is characterized byhaving an exterior layer of dense, fine grained, low-carbon steel. Sucha tube can be collapsed to a plate and roll-welded on its interiorcontiguous surfaces to yield a product capable of being rolled to sheetstock which exhibits all of the properties (smooth surface, highpolish-ability, and deep drawing characteristics) required of automotivesheet stock. Such a tube can also be slit longitudinally and flattenedto plate stock, by prior art processes, and rolled to sheet having thedesired properties on one (the tubes exterior) surface.

It can be appreciated that such automotive sheet stock can also beproduced from batch-type centrifugally cast cylinders of steel by theexpedient of an extended (slow) cooling action using pre-heated or lowheat conductivity molds of a solid wall nature.

The extended-hotzone is basically a means of slowing the solidificationrate over a specific temperature range. With low-carbon steel this rangecoincides with the deltaferrite region of the iron-carbon phaseV diagramwhich encompasses the temperature range of about 1500 to 1475o C.

The extended-hot-zone (slowed solidication range) can, by intentionallyvarying the length of the hot-zone or 'utilizing higher G forces, createa wide variation of surface properties in collapse-formed sheet productsmade from such tube. Ordinarily, the extended-hot-zone is used onlywhere an end product of uniquely advantageous properties is created (asautomotive sheet stock). The hot zone is restricted to that necessaryfor leveling or smoothing of the molten steel or other material layerunder all other conditions. This is especially true where the tube is tobe longitudinally collapse-formed to a structural item (as I-beam orrailroad rails) where a lower carbon surface could result in a loss offatigue resistance.

Other alloys can be advantageously processed by the technique of usingan extended-hot-zone. Cast iron pipe continuously centrifugally castfrom gray or nodular irons can be produced with a gradient metallurgicalstructure (from the exterior to interior surface of the pipe) of varyingcarbon content which exhibit advantageous properties under certainconditions of use. Silicon steels can be so treated to produce ahighsilicon interior surface on the centrifugally cast tube.

I claim:

1. A method for continuously casting tubing from molten casting materialcomprising the following steps:

rotating on a generally horizontal axis, an elongated tubular moldhaving an inlet at one end, an outlet at its other end, and a solidcylindrical casting charnber between said inlet and outlet;

injecting said molten casting material through said inlet into saidcylindrical casting chamber and causing it to assume the form of acylindrical tube in response to the rotation of said mold, and causingsaid tube of liquid casting material to be cooled and to thereby becongealed to a state within the range including solidified andsemi-solidified states;

causing controlled output of said tube from the mold;

and controlling the sidewall pressure of the casting material within themold so as to restrict the sidewall frictional forces between thecasting material and the mold wall which normally oppose the exit of thetube from the mold;

wherein the restriction of said sidewall frictional forces is effectedby subjecting said tube to a differential of external gas pressure overinternal gas pressure.

2. The method defined in claim 1, wherein said pressure differential isdeveloped by applying vacuum to the interior of said tube whilesubjecting the exterior of the tube to atmospheric pressure.

3. The method defined in claim 2, including the step of injecting aninert purge gas into said tube while applying said vacuum thereto.

4. The method defined in claim 1, wherein said restriction of sidewallfrictonal forces is effected by applying a vacuum to said tubeinternally thereof while subjecting the tube externally to atmosphericpressure;

and collapse-sealing the exiting portion of the tube to maintain saidvacuum.

5. The method defined in claim l, wherein said restriction of sidewallfrictional forces is effected by application 21 of supra-atmosphericpressure to said tube externally thereof.

6. The method defined in claim 1, wherein said restriction of sidewallfrictional forces is effected by application of supra-atmosphericpressure to the exterior of said tube within said mold;

sealing said pressure within the mold around said inlet;

and sealing the exterior of the exiting tube to said outlet.

7. A method for continuously casting tubing of a material of apredetermined specific gravity and melting temperature, comprising thefollowing steps:

rotating on a generally vertical axis, an elongated tubular mold havingan inlet at the top end and an outlet at its bottom end; providing acylindrical casting chamber within the mold; injecting said castingmaterial in molten form into said cylindrical casting chamber andcausing it to assume the form of a cylindrical tube in response to therotation of the mold, and causing said tube of liquid casting materialto be cooled to a solidified state;

restricting the sidewall frictional forces of said tube against the moldwall by subjecting said tube to a gas pressure differential operative todecrease said frictional forces;

and causing controlled output of said tube;

the restriction of said frictional forces facilitating said output.

8. The method defined in claim 7 wherein said restriction of sidewallfrictional forces is effected by creating a vacuum within said tube.

9. The method defined in claim 7 wherein said restriction of sidewallfrictional forces is effected by applying supra-atmospheric pressure tothe exterior of the discharging tube.

10. The method of claim 1 wherein:

said pressure differential is effected by causing a partial vacuum inthe interior of said tube.

11. The method of claim 1, wherein:

said pressure differential is effected by applying supraatmosphericpressure to said tube externally thereof and a partial vacuum internallythereof.

12. The method of claim 7, wherein:

the restriction of said sidewall frictional forces is effected byapplying supra-atmospheric pressure to the exterior of the exiting tubeand creating a partial vacuum within said tube.

13. A method for continuously casting tubing from molten castingmaterial comprising the following steps:

rotating a generally tubular mold having an inlet and an outlet,

introducing said molten casting material into said inlet,

said molten casting material assuming the shape of a tube in response tothe centrifugal forces produced by the rotation of said mold,

causing said tube to exit from said outlet in a relatively solidifiedstate,

producing a gas pressure inside said tube sufficiently lower than thepressure outside to contract said tube enough to reduce substantiallythe sidewall frictional forces tending to oppose exit of said tube fromsaid mold,

sealing the inlet end of said mold by means of a nonrotating end platewhose periphery is immersed in a centrifugal annular trough containing arelatively dense liquid, and

sealing the exit end of said tube,

whereby said sealing steps serve in maintaining said gas pressure.

References Cited UNITED STATES PATENTS 777,559 12/1904 Straus et al164-84 2,940,143 6/1960 Daubersy et al 164-5 3,367,400 2/ 1968 Hathorn164-84 FOREIGN PATENTS 22,708 ll/l896 Great Britain 164-81 R. SPENCERANNEAR, Primary Examiner

